Somewhere along the telephone line there seems to be a little confusion of the stages of jiangshi. There are 6 that you know of. 5 that you initially figured out on your own through knowledge checks and one more mentioned by the professor. They are:

1) white jiangshi - weak, covered in white hairs, afraid of everything. Pushing them over is enough to kill them. They feed on rodents.2) black jiangshi - white hairs turn black, no longer afraid of larger animals (which they now feed on), still afraid of people. Fire/sunlight kills them. 3) jumping jiangshi - the main kind of jiangshi. limbs become stiff so they have to jump instead of walk. Feed on people. They don't like sunlight but it doesn't kill them anymore, nor does fire. Only peach wood does. 4) flying jiangshi - They don't jump anymore, they fly. They also don't need to touch you anymore to feed.5) ba - demigods who can shapeshift into any creature, cause droughts, and infect large groups of people with plague. They can even feed on dragons if they wish. 6) demon king - biggest, baddest jiangshi. Only one known to ever exist and it takes thousands of years to become one. It is a god with limitless powers whose appearance means the end of the world if not stopped

So while you don't know how long it takes to turn into a ba, the next stage is flying jiangshi. While a flying jiangshi is bad news, it is far less so than a ba. In the very least it will take years for it to grow into a ba unless it has an absurd amount of qi available to feed on. Nothing about it's current feeding pattern has suggested that it is absorbing qi at such an accelerated rate though.